This invention is used to inspect storage racks for spent nuclear fuel before their installation and use.
One of the problems which electric power companies have to resolve when they use a nuclear reactor for generation of power is the disposition of the spent nuclear fuel. A fuel assembly may have a useful life of 4 years, at the end of which it must be replaced. Storage racks have been developed for storage of spent fuel while awaiting final shipment off site. Such racks are placed in a pool of water in the reactor building and kept completely submerged for shielding and cooling. The pool of water is so located relative to the reactor that the spent fuel assemblies may be lifted, as by a crane, and placed in one of the storage locations in the rack. Since water is an excellent shield and coolant, protection is afforded during both the removal of the spent fuel assembly from the reactor assembly and also during the storage thereof in the submerged rack.
"High density" racks differ from normal storage racks in that they use plates of a thermal neutron absorbing material between the stored fuel assembles to decrease the reactivity of the storage configuration, thus permitting closer spacing between fuel assemblies, or a higher density of storage. Therefore, the capacity of the spent fuel storage pool is increased. Because the thermal neutron absorber is crucial in preventing an accidental criticality of the closely spaced fuel within the racks, the Nuclear Regulatory Commission is requiring utilities installing these racks to verify the presence of all the thermal neutron absorber plates within these racks. Since the plates are not visible inside the racks (because they are contained between the walls of two concentric square tubes), it is necessary to perform this inspection using a neutron source and detector.
The four thermal neutron absorber plates sealed within the two concentric square tubes form a "poison can." The poison cans are placed in every other storage location of a high density rack (checkerboard fashion). When a fuel assembly is inserted into a location which has a poison can, each of the four poison plates in the poison can will be adjacent to one of the four sides of the fuel assembly. When another fuel assembly is inserted into an adjacent storage location, a poison plate lies between the two assemblies, preventing thermal neutrons from passing and, therefore, chain reaction from occurring. A material called Boral (a product of Brooks and Perkins Inc.) is often used for the poison plates. Boral is made of boron carbide powder (boron being an excellent thermal neutron absorber) dispersed in ann aluminum matrix, which is sandwiched between two sheets of thin aluminum cladding. The resulting plate is one eighth (1/8") inch thick, and four such plates approximately five and three fourths (53/4") inches by thirteen (13') feet are sealed between two concentric square aluminum tubes, each about one eighth (1/8") inch thick, to form the above mentioned poison can. Each high density rack contains from 64 to 120 fuel storage locations in a rectangular matrix; since poison cans are only placed in alternate storage locations, a rack may contain from 32 to 60 poison cans. There are different high density rack designs which may or may not use poison cans as described herein.